Note: Descriptions are shown in the official language in which they were submitted.
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MET7~0D OF DRILLINC A DIRECTIO~AL ~J~LL B9~E
The invention relates to a method of drilling a directional ~el.
bore, usually in order to produce a fluid, such as oil and/or
gas, contained in an underground formation.
Many oi] or gas wells are not drilled vertically but wlth a
certain angle or inclination to vertical. The target location,
determined before drilling, does not lie vertically below the
surface location of the drilling rig. This is particularly true
when drilling offshore when a cluster of wells is drilled from
the same rig. The majority of these deviated wells are of the
"build and tangent" type, depicted in Figure 1. From the rig
located at the surface S, the well is first drilled downwards
vertically to a prescribed depth Dl. Then, the well trajectory
kicks off and the angle of inclination to vertical is built,
ideally at some fixed rate, to some predetermined angle ~ formed
between a vertical line and the longitudinal axis of the well
bore. This part of the borehole is called the build section.
Then, the hole is drilled straight at the target T in the oil or
gas producing formation F, maintaining the inclination angle as
close to ~ as possible ~mtil the target is reached. This last
part of the hole is called the tangent section.
The drilling assembly, or drill string, used to drill a well is
mainly composed of a pipe string with a drilling bit at its lower
end and drill collars located just above the bit. Drill collars
are heavy tubes (compared with drill pipes), used to put weight
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on the drill bit. Usually, all the available wei~ht i8 not
applied to the bit, i.e. the drill string is retained at the
surface. Consequently, the upper part of the drill string is
under tension and the lower part is under compression. The point
in-between, where the stress changes rom tension to compression
is the neutral point which is usually located in the upper part
of the drill collars section.
However, for deviated wells, the hook load when drawing the drill
string out or the hole (tripping out) is substantially greater
than the free (rotating) weight of the string. In addition, the
torque required at the surface to achieve a given (lower) torque
at the bit is substantially greater in the case of a aeviated
well than in the case of a vertical well of similar length.
In general, drag and torque loss in a drill string system are
associated with the side forces acting along the drill string
giving rise to a frictional interaction between the string and
the well bore. The side forces are comprised of two components
depicted in Figure 2 and associated with : -
- the local curvature c of the string (which is taken to lie
in a vertical plane) giving rise to a term T.c where T is
the local tension and
- the component of the buoyed mass of the string acting
orthogonally to the tangent to the tra~ectory. This gives
rise to a term of the form mgsin (~) where ~ is the
inclination angle and m the buoyed mass of the drill string
per unit length.
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The total contribution of these two terms to the drag or the
torque loss is given by a term depending on the coefficlent of
friction of the form :
~ ¦mgsin (~) - Tc¦ '
integrated over the entire length of the string.
In certain circumstances, particularly in long reach wells, the
induced drag can be of such a magnitude that the drillillg process
is hindered. This can occur either because it becomes difficult
or impossible to trip out or because the torque required to
rotate the drill string exceeds the rating of the rotary table.
US patent no. 4,44~,241 describes a method of drilling a well
bore that substantially reduces the likelihood of the drill
string becoming stuck and reduces the frictional forces between
the drill string and the well bore. According to this method, the
well bore is drilled along the path of a catenary curve. However,
this method is very difficult to implement, because for a
catenary curve, the variation of the inclination angle is not
constant but has to increase continuously. In practice, drilling
a borehole along a catenary path ls an impossible task. For
instance, if two stabilizers are used to deviate the trajectory
of the borehole, the distance between the two stabilizers has to
be increased regularly in a predetermined way. This i8 not easily
achleved and it requires fine control from the directional
driller. In addition, frequent correction runs to return the
trajectory to catenary could readily give rise to regions of
local dog legs which, in turn, would increase dra8 and torque.
Another drawback of the method is that the inclination of the
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borehole ~hen reaching the target location is of~en Yery l~rgo:
the borehole lies nearly horizontally. This large inclination
might not be appropriate with an effici.ent production of tho for-
m~tion fluid. It also increases the dray of the bottorn h~le
assembly and therefore the side forces acting on the boreh~le
string, making worse the problems of borehole stability and
.stahillzer stickingt
The primary object of the invention is to provide a
method of drilling a well bore that substantially reduces the
drag and t^rque loss in the drill string system and that can be
implemented easily.
According to the present invention there is provided
an improved method of drilling a directi.onal well horeho].e with
a drill string, along a predetermined trajectory eYtending
between a starting location at the surface and an underground
final depth point horizontally and vertically displaced from said
starting location, said method somprising the steps of:
(1) drilling a first, substantially vertical section of said
borehole under said starting location;
(2) drilling a second section of said borehole having a
substantially constant build rate, said second section
immediately preceding said final depth point; and
(3) drilling a third section of said borehole having a sub-
stantially constant bu.ild rate, said third section being
for~,ed at the end of said first section between said first ~nd
second sections, and said third section having a build rate
substantially greater than that of said second section, and a length
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su~stantially smaller than that of said second section,
In order that features and advantayes of the presen
invention may be appreciated, an e~ample will now ,~e d~scri~ed
with reference to the accompanying diagrammati,c dra~"inys of
which:
Figure 1 represents the trajectory of a well drill~d in
accordance with the prior art;
Figure 2 represents the forces actiny on a section o~ a
drill string;
Figure 3 shows the trajectory of a borehole drilled
according to the invention;
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- Figure 4 shows a practical example of a well bore drilled
according to the method of the invention, and
- Figure 5 and 6 show the variatlon respectively of the noo~
load when tripping out and of the torque as a function of
the angle at the end of the initial build sect-on for a
constant build trajectory.
The ai~ of the proposed method is to reduce the drag and torque
loss experienced in most of the directional wells.
There ~re mainly two means of ameliorating the drag problems of a
well. The first is to counter some of the load force in the
tan~ent section while the second is to reduce the eYtent of the
bulld section. The secor.d of these is important since the build
section is high in the drill string, tension is consequently
large and the side force and associated drag is high in this
region. Reduction of the side forces not only r~duces drag but
also reduces the wear on the casing (the steel tube which linec
the well bore).
The method of the present invention combines both of the options
outlined above. First, the conventional tangent section ~also
called "hold section") depicted in Figure 1 is replaced by a
constant (upward) curvature section to target. Second, the
initial build section is reduced in extent so that the angle
achieved at the end of the initial build section is lower than
that required for a conventional build/tangent well. This
reduction of the initial build section is the consequence of the
use of a constant curvature section for the last part of the
borehole.
In practlce, the building characteristics of a well trajectory
are achieved by the strategic placement of stabilizers in the
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bottom h~'e assembly of the drill string In general, a given
bottom hole assembly, at constant weight on bit, ~ill tend ~o
build angle at a fairly constant rate. In order to change
slightly the inclination of the borehole, the driller modifies
the weight on bit. For a substantial change of inclir.ation, the
driller has to modifiy the distance between the stabilizers. The
drill string is therefore tripped out, the stabilizers positions
in the borehole assembly is modified and the drlll string lowered
again in the borehole to resume the drilling operation.
The method for drilling a constant build trajectory well is
illustrated on Figure 3.
The initial vertical section 12 is drilled from the rig R ~o the
aesired detph l at which point 14 the well kicks off. The initial
build section 16 is then drilled at a build rate b (degrees per
hundred feet) generating an arc of radius r1 where
18000
rl
~ b
The initial build section is continued until point 18, where
some pre-determined inclination angle ~ is achieved. In general,
the initial build section 16 will be a necessary requirement as
it serves two purposes : to clear neighbouring wells as quickly
as possible, in the case of high density of wells, such as for
cluster wells, and to define an initial compass bearing for the
well. The driller needs, as a matter of fact, to determine fairly
quickly the azimuth of the borehole. This last requirement will
normally constrain 0 to take some value greater than about
15 - 20. Notwithstanding these comments, a well with no initial
build section can be planned by taking 0 = 0 in the following
formulae.
At the end 18 of the initial build section, the vertical depth v
is given by :
v = 1 + rly sin 0
and a horizontal displacement d glven by
d - rl (1 - Cos 0)
For a well with a target (at some vertical depth Yt and some
horizontal displacement xt the quantities ~ x and ~y are defined
bv :
~ x = xt - d
and
~ y = y - v
The constant build traiectory 20 from the end 18 of the initial
build section 16 to the target T (with matching tangent at the
end of the initial build section) is given by :
(x - d _ -X)2 = (y - v _ y)2 = R
where x and y are the horizontal and vertical components relative
to the rig location, and where :
X = (~Y + ~K )2 Cot
2 2~y 1 +~x Cot
~y ~ x) 2 ~ _
2 2~y ~y
The radius of curvature R is given by : R = (x + y )
t.~
To achieve this trajectory in practice, an appropriate '~ottom
hole assembly is run at the end of the initial bulld section and
the well is caused to build angle constantly at a rate of 18000/R
degrees per hundred feet until the target ii reached. Por a
typical well, this value of the build rate would be between 0,2
and 05 per 100 feet.
Calculations of the total hook load, whe~ tripping out from full
depth, and of the rotary torque were made for a typical modeI,
well shown in Figure 4, to exhibit the possible reduction in drag
and torque loss gained by using curved trajectories. The well is
drilled vertically to a kick off point 30 at 2400 feet. The
inclination was then build at a rate of 5 per 100 feet to some
angle ~ at point 32. This angle would be typically between 2 and
8 per 100 feet. The target T was at a total vertical depth of
9000 ft with a step out fron the rig of 6000 feet. Drilled as a
conventional build and hold trajectory (such as the well
tra,ectory shown on Figure 1) this would correspond to an
inclination angle of 44.5.
The model drill string was configured with 372 feet of 6 1/2 inch
drill collar and 840 feet of 5 inch heavyweight pipe with 5 inch
drill pipe to surface. A mud weight of 9.8 lb per gallon was
used. The drag and torque loss are a function of the coefficient
of friction and this would normally be expected to lie in the
range 0.2 - 0.4. In this example, a value of 0.4 was used to
simulate harsh drag condit~ons. The torque loss calculation was
made assuming a weight on bit of 38000 lb.
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Figure ; shows J for this model well, the hook load in l~Klb whe
tripping ~ut from full depth as a function of the angle ~ a~ the
end of the 5 per 100 foot section, between point3 30 and 32. rne
upper curve 34 is the hook load for the constant curva~ure
trajectory while the lower curve 36 depicts the hook load for a
catenary trajectory. The two curves 34 and 36 are virtually
co_ncident for inclination angles above 30. '~ith a conventional
trajectory (0 = 44.5), a hook load of about 32~ Klb would be
expected. For a curved section well with g = 30, both the
catenary and the constant build trajectory reduce this figure by
about 55 ~g lb.
Figure 6 shows the rotary torque as a function of ~ for a well
bore drilled according to the present invention. For the
conventional trajectory, the torque loss from the surface to the
bit is in the region of 22,500 foott lb while the constant build
trajectory from inclinations of about 30 reduces this loss by
abGut 4,500 foot lb.
~'hile it has been shown and described in Figure 3 what is
considered to be the preferred embodiment of the invention, it
will be apparent to those skille& in the art that various changes
and modifications may be made therein without departing fro~ the
spirit or scope of the invention.
